1. Field of the Invention
The present invention relates to an optical disc on which a land and groove are formed as well as a recording/reproduction method onto/from this optical disc.
2. Description of the Prior Art
As an optical disc for recording and/or reproducing an information signal by laser beam radiation, there are known a magneto-optical disc, a phase change type optical disc, writable optical disc, digital audio disc (so-called compact disc), optical video disc (so-called laser disc), and the like. These optical discs include a transparent substrate made from a resin such as polycarbonate or glass having function films such as a recording layer and reflection layer. Moreover, these optical discs are used like a ROM (read only memory) for carrying out only reproduction such as a compact disc and a laser disc or like RAM (random access memory) capable of reproduction, additional writing, and rewriting.
On these optical discs, a groove is formed as a guide groove for tracking servo. Moreover, in these optical discs, a convex portion between adjacent grooves is called a land. The optical discs can be divided into a groove recording type in which an information signal is recorded in the groove, and a land recording type in which an information signal is recorded on the land.
The phase change type optical disc is used like RAM and need not provide means for generating an external magnetic field. Accordingly, a recording/reproduction apparatus of the phase change type optical disc can be made with a small size and an overwrite of an information signal can easily be carried out, which currently attracts a special attention.
In this phase change type optical disc, a predetermined laser beam is applied to a recording layer so that a material constituting the recording layer of the portion subjected to the laser beam reversibly changes its crystal state and the amorphous state. Recording and reproduction are carried out by detecting a change in reflectivity caused of a difference of optical constants in these states.
More specifically, in this phase change type optical disc, firstly, the recording layer in a crystal state has an initial state. When an information is recorded on this phase change type optical disc, a laser beam having a strong power is applied to a recording layer. The portion subjected to this laser beam increases the temperature above a melting point and after this, the portion is cooled down to an amorphous state.
Moreover, in this phase change type optical disc, when erasing an information signal recorded on the recording layer, a laser beam weaker than the laser beam used for recording is applied so that the recording layer increases its temperature above the crystallization temperature but below the melting point. As a result, the portion subjected to this laser beam becomes a crystal state regardless of the preceding state.
Furthermore, in this phase change type optical disc, when reproducing an information signal recorded on the recording layer, a laser beam is applied to the recording layer. Here, the reflectivity of the laser beam changes depending on whether the portion subjected to the laser beam is in the crystal state or the amorphous state. By detecting this reflectivity, an information signal is reproduced.
Now, there is a DVD-ROM (digital video disc--read only memory) having a diameter identical to that of a compact disc and 4.7 GB recording capacity on one side. In contrast to this, there is a DVD-RAM (digital video disc--random access memory) using a phase change type disc having 2.6 GB capacity on one side. The DVD-ROM and the DVD-RAM have the same specifications for recording and/or reproduction associated with the data transfer rate, the modulation method, and the error correction method.
However, the DVD-RAM using the aforementioned phase change type optical disc employs the land-groove method, whereas the DVD-ROM employs an emboss-pit method like in the compact disc.
For reproducing the DVD-ROM using the groove recording method, a DVD-ROM reproduction apparatus used has a 650 nm wavelength red semiconductor laser and an objective lens of numerical aperture NA=0.6. This DVD-ROM reproduction apparatus cannot reproduce a DVD-RAM using the aforementioned phase change type optical disc.
Thus, the conventional optical disc has a problem that a reproduction-dedicated optical disc reproduction apparatus cannot reproduce a writable optical disc.
Moreover, in the case of the aforementioned DVD-RAM which employs the land-groove recording method, an emboss-pit is formed between the land and the groove. This emboss pit is formed to switch between the land the groove and should be formed with a high accuracy. Accordingly, the aforementioned rewritable optical disc requires a high-accuracy cutting technique in the mastering step. Consequently, such a rewritable optical disc has a difficulty in a production procedure.